Hmmm... It seems to me that your sister is doing too much too soon. In my opinion, she's not ready to be cantering. It's dangerous and uncomfortable for both her and the horse. At the moment, any critique would be pretty pointless because of the stage she's at. She has low confidence and lack of experiance, and 2 or 3 times a year isn't enough to give her the experiance she needs to ride effectively. Before she can start with cantering and jumping, she needs to backtrack and work on the basics.
For your sister I'd sugest some more constant but lighter riding lessons. Nothing more than walk/trot and some ground poles, and perhaps a little bareback. She needs a good qualified instructor, one who will help her perfect the basic skills so that she has the confidence to move forwards when she's ready. She'll both benefit from and enjoy this method a lot more, and I'm sure she'd be more comfortable doing it this way.
As for you, I can tell that the saddle is definitely too small, so I'm going to cut you some slack.
At the trot, I think your leg can come back a tad so that it is just under your hip. The horse you're riding is a little lazy, am I wrong? When riding him, you try to push him forward by standing higher and forcing your seat up while you post, which honestly doesn't do much for you. Instead, stay low to the saddle and post forward, back
instead of just up, down
. Imagine a string tied to your belly button that pulls you forward every time you rise out of the saddle (that`s how I learned to post
At the canter, you seemed to be tipping back a lot. You also are absorbing the weight with your body instead of your legs and core. Jam your heels down and move your leg back - this will help you have a quieter seat and upper body. I know the saddle is small, so it would benefit you to ride in half seat instead of full seat.
Good luck to both you and your sister.